Roller conveyors, whether rigid frame or of lazy tong structure, are conventional. For instance, U.S. Pat. No. 4,852,712 issued Aug. 1, 1989 to Best entitled "Conveyor" discloses an extensible lazy tong conveyor. U.S. Pat. No. 5,224,584 issued Jul. 6, 1993 to Best, et al. entitled "Expandable Powered Conveyors" discloses power conveyors in which the power units are located external to the rollers and connected to them via a number of chains or belts. U.S. patent application No. 08/022,012 to Best, et al. filed Feb. 24, 1993 entitled "Controllably Powered Roller Conveyors" discloses powered lazy tong conveyors in which the power units are located within the rollers. All of these patent documents are incorporated by this reference.
Additionally, various forms of rigid frame roller and skate wheel conveyors have been used for many years for various purposes. In the past, however, in the event a rigid frame conveyor needed to be extensible, the rollers in one frame segment which could telescope into another frame segment must be positioned at a different height than the rollers in adjacent segments, in order for nesting to be possible. Such variations in height may cause conveyor jams as articles such as boxes drop from one segment to another and their corners become lodged between rollers. Accordingly, conventional resolution has been to include non-roller transitions in the form of short ramps which allow the articles on the conveyor to slide from one level of roller to another level of roller on another segment. Even measures such as this cannot avoid jamming ultimately, however, because the distance between the end of the ramp and the rollers on the lower level is variable and prone to catching box corners.
Additionally, recent ergonomic requirements, occupational health and safety requirements, and greater concern for the welfare of workers have created the need for expandable conveyors to operate in a powered, automatic and easily controllable fashion. Not only must the conveyor contain reliable power means for causing it to extend and retract, but it must also include limit and delimit switches in order to avoid overextension and for safety concerns.